The present invention relates generally to methods for making paper products. More particularly, the invention concerns methods for making cellulosic webs having high bulk and absorbency on a modified conventional wet-pressed machine.
There are generally two different methods for making the base sheets for paper products such as paper towels, napkins, tissue, wipes and the like. These methods are commonly referred to as wet-pressing and throughdrying. While the two methods may be the same at the front end and back end of the process, they differ significantly in the manner in which water is removed from the wet web after its initial formation.
More specifically, in the wet-pressing method, the newly-formed wet web is typically transferred onto a papermaking felt and thereafter pressed against the surface of a steam-heated Yankee dryer while it is still supported by the felt. As the web is transferred to the surface of the Yankee dryer, water is expressed from the web and is absorbed by the felt. The dewatered web, typically having a consistency of about 40 percent, is then dried while on the hot surface of the Yankee dryer. The web is then creped to soften it and provide stretch to the resulting tissue sheet. A disadvantage of wet pressing is that the pressing step densifies the web, thereby decreasing the bulk and absorbency of the tissue sheet. The subsequent creping step only partially restores these desirable sheet properties.
In the throughdrying method, the newly-formed web is first dewatered using vacuum and then transferred to a relatively porous fabric and non-compressively dried by passing hot air through the web. The resulting web can then be transferred to a Yankee dryer for creping. Because the web is substantially dry when transferred to the Yankee dryer, the density of the web is not significantly increased by the transfer. Also, the density of a throughdried sheet is relatively low by nature because the web is dried while supported on the throughdrying fabric. The disadvantages of the throughdrying method are the relatively high operational energy costs and the capital costs associated with the throughdryers.
Because the vast majority of existing tissue machines utilize the older wet-pressing method, it is of particular importance that manufacturers find ways to modify existing wet-pressed machines to produce the consumer-preferred low-density products without expensive modifications to the existing machines. Of course, it is possible to re-build wet-pressed machines to throughdried configurations, but this is usually prohibitively expensive. Many complicated and expensive changes are necessary to accommodate the throughdryers and associated equipment. In addition, the length of a through-air dried tissue machine is greater, requiring a building addition or modification. In some locations, building modifications are not practical or possible, or prohibitively expensive because of the interference with other existing equipment or limited area available on the site. Accordingly, there has been great interest in finding ways to modify existing wet-pressed machines without significantly altering the machine design.
In some instances, it is more convenient and cost effective to modify the press section of a wet-pressed tissue machine than the wet end, particularly if the wet end and headbox are in good condition. In addition, older wet-pressed machines may have existing equipment associated with the bottom felt run that can be readily adapted for other uses, making the modification simpler and even more cost effective. This invention discloses simplified methods of modifying a wet-pressed machine to make an improved consumer-preferred low-density product.
One simple approach to modifying a wet-pressed machine to produce softer, bulkier tissue is described in U.S. Pat. No. 5,230,776 issued Jul. 27, 1993 to Andersson et al. The patent discloses replacing the felt with a perforated belt of wire type and sandwiching the web between the forming wire and this perforated belt up to the press roll. The patent also appears to disclose additional dewatering means, such as a steam blowing tube, a blowing nozzle, and/or a separate press felt, that may be placed within the range of the sandwich structure in order to further increase the dry solids content before the Yankee dryer. These extra drying devices are said to permit the machine to run at speeds at least substantially equivalent to the speed of throughdrying machines.
It is important to reduce the moisture content of the web coming onto the Yankee dryer, to maintain machine speed and to prevent blistering or lack of adhesion of the web. Referring to U.S. Pat. No. 5,230,776, the use of a separate press felt, however, tends to densify the web in the same manner as a conventional wet-pressed machine. The densification resulting from a separate press felt would thus negatively impacting the bulk and absorbency of the web.
Further, jets of air for dewatering the web are not per se effective in terms of water removal or energy efficiency. Blowing air on the sheet for drying is well known in the art and used in the hoods of Yankee dryers for convective drying. In a Yankee dryer hood, however, the vast majority of the air from the jets does not penetrate the web. Thus, if not heated to high temperatures, most of the air would be wasted and not effectively used to remove water. In Yankee dryer hoods, the air is heated to as high as 900 degrees Fahrenheit and high residence times are allowed in order to effectuate drying.
Thus, what is lacking and needed in the art is a practical method for making tissue sheets having high bulk and absorbency comparable to throughdried sheets on a modified, conventional wet-pressed machine.
It has now been discovered that a wet-pressed tissue can be made having bulk and absorbency properties equivalent to those of comparable throughdried products, while maintaining reasonable machine productivity. More particularly, wet-pressed cellulosic webs can be made by vacuum dewatering a wet web up to approximately 30 percent consistency, then using an integrally sealed air press to noncompressively dewater the sheet to 30 to 40 percent consistency. The wet web is desirably then transferred to a xe2x80x9cmoldingxe2x80x9d fabric substituted for the conventional wet-pressing felt in order to impart more contour or three-dimensionality to the wet web. The wet web is preferably thereafter pressed against the Yankee dryer while supported by the molding fabric and dried. The resulting product has exceptional wet bulk and absorbency exceeding that of conventional wet-pressed towels and tissue and equal to that of presently available throughdried products.
As used herein, xe2x80x9cnoncompressive dewateringxe2x80x9d and xe2x80x9cnoncompressive dryingxe2x80x9d refer to dewatering or drying methods, respectively, for removing water from cellulosic webs that do not involve compressive nips or other steps causing significant densification or compression of a portion of the web during the drying or dewatering process.
The wet web is wet-molded in the process to improve the three-dimensionality and absorbent properties of the web. As used herein, xe2x80x9cwet-moldedxe2x80x9d tissue sheets are those which are conformed to the surface contour of a molding fabric while at a consistency of about 30 to about 40 percent and then dried by thermal conductive drying means, such as a heated drying cylinder, as opposed to other drying means such as a throughdryer, before optional additional drying means.
The xe2x80x9cmolding fabricsxe2x80x9d suitable for purposes of this invention include, without limitation, those papermaking fabrics which exhibit significant open area or three-dimensional surface contour sufficient to impart greater z-directional deflection of the web. Such fabrics include single-layer, multi-layer, or composite permeable structures. Preferred fabrics have at least some of the following characteristics: (1) On the side of the molding fabric that is in contact with the wet web (the top side), the number of machine direction (MD) strands per inch (mesh) is from 10 to 200 (3.94 to 78.74 per centimeter) and the number of cross-machine direction (CD) strands per inch (count) is also from 10 to 200 (3.94 to 78.74 per centimeter). The strand diameter is typically smaller than 0.050 inch (1.27 mm); (2) On the top side, the distance between the highest point of the MD knuckle and the highest point of the CD knuckle is from about 0.001 to about 0.02 or 0.03 inch (0.025 mm to about 0.508 mm or 0.762 mm). In between these two levels, there can be knuckles formed either by MD or CD strands that give the topography a 3-dimensional hill/valley appearance which is imparted to the sheet during the wet molding step; (3) On the top side, the length of the MD knuckles is equal to or longer than the length of the CD knuckles; (4) If the fabric is made in a multi-layer construction, it is preferred that the bottom layer is of a finer mesh than the top layer so as to control the depth of web penetration and to maximize fiber retention; and, (5) The fabric may be made to show certain geometric patterns that are pleasing to the eye, which typically repeat between every 2 to 50 warp yarns.
Hence, in one aspect, the invention resides in a method for making a cellulosic web, comprising: (a) depositing an aqueous suspension of papermaking fibers onto an endless first fabric to form a wet web; b) dewatering the wet web to a consistency of about 10 percent to about 30 percent; c) transferring the wet web to an endless second fabric; d) sandwiching the wet web between the second fabric and a support fabric and dewatering the wet web to a consistency of greater than 30 percent using a noncompressive dewatering device that is adapted to cause a pressurized fluid at about 5 pounds per square inch gauge or greater to flow substantially through the web due to an integral seal formed with the wet web; (e) pressing the dewatered wet web against the surface of a heated drying cylinder to at least partially dry the wet web; and, (f) drying the dewatered wet web to a final dryness.
In another aspect, the invention resides in a method for making a cellulosic web, comprising: (a) depositing an aqueous suspension of papermaking fibers onto an endless first fabric to form a wet web wherein the wet web; (b) transferring the wet web to an endless second fabric; (c) sandwiching the wet web between the second fabric and a support fabric and dewatering the wet web to a consistency of up to about 30 percent; (d) supplementally dewatering the wet web to a consistency of about 30 to about 40 percent using an air press that is adapted to cause a pressurized fluid at about 5 pounds per square inch gauge or greater to flow substantially through the web due to an integral seal formed between an air plenum and a collection device; (e) configuring the second fabric to provide an unsupported sheet wrap angle of the dewatered wet web about a pressure roll of less than 90 degrees; (f pressing the dewatered wet web against the surface of a heated drying cylinder to at least partially dry the dewatered wet web; and, (g) drying the dewatered wet web to a final dryness.
In another aspect, the invention resides in a method for making a cellulosic web, comprising: (a) depositing an aqueous suspension of papermaking fibers onto an endless first fabric to form a wet web; (b) dewatering the wet web to a consistency of up to about 10 percent; (c) transferring the wet web to an endless second fabric; (d) sandwiching the wet web between the second fabric and a support fabric; e) passing the wet web sandwiched between the second fabric and the support fabric between an air plenum and a collection device with the second fabric disposed between the wet web and the collection device, the air plenum and collection device being operatively associated and adapted to create a pressure differential across the wet web of about 30 inches of mercury or greater and a stream of pressurized fluid through the wet web of about 10 standard cubic feet per minute per square inch or greater; f) dewatering the wet web using the stream of pressurized fluid to a consistency of about 30 percent to about 40 percent; g) pressing the dewatered wet web against the surface of a heated drying cylinder with the second fabric; and, (h) drying the dewatered wet web to a final dryness.
In another aspect, the invention resides in a method of modifying a conventional wet press machine having at least one felt and compressive dewatering devices, comprising: (a) replacing at least one felt with at least one fabric; and, (b) replacing compressive dewatering devices with non-thermal, noncompressive dewatering devices. Wet press processing and equipment are discussed in U.S. Pat. No. 4,139,410 issued to Tapio et al. on Feb. 13, 1979 and incorporated herein by reference.
In another aspect, the invention resides in a method for making a cellulosic web, comprising the steps of: (a) depositing an aqueous suspension of papermaking fibers onto an endless first fabric to form a wet web; (b) dewatering the wet web to a consistency to about 10 to about 30 percent; (c) transferring the wet web to an endless second fabric; (d) sandwiching the wet web between the second fabric and a support fabric and dewatering the wet web to a consistency of greater than about 30 percent using a non-compressive dewatering device that is adapted to cause a pressurized fluid at about 5 pounds per square inch gauge or greater to flow substantially through the wet web due to an integral seal formed with the wet web; (e) transferring the wet web back to the second fabric; (f) pressing the dewatered wet web against the surface of a heated drying cylinder to at least partially dry the wet web; and, (g) drying the wet web to a final dryness.
In another aspect, the invention resides in a method for making a cellulosic web, comprising the steps of: (a) depositing an aqueous suspension of papermaking fibers onto an endless first fabric to form a wet web; (b) transferring the wet web to an endless second fabric; (c) sandwiching the wet web between the second fabric and a support fabric and dewatering the wet web to a consistency to about 30 percent; (d) further dewatering the wet web to a consistency of greater than about 30 percent to about 40 percent using an air press that is adapted to cause a pressurized fluid at about 5 pounds per square inch gauge or greater to flow substantially through the wet web due to an integral seal formed between an air plenum and a collection device; (e) transferring the wet web back to the second fabric such that the sheet wrap of the wet web on the pressure roll is less than 90xc2x0; (f) pressing the dewatered wet web against the surface of a heated drying cylinder to at least partially dry the wet web; and, (g) drying the wet web to a final dryness.
In yet another aspect, the invention resides in a method for making a cellulosic web, comprising the steps of: (a) depositing an aqueous suspension of papermaking fibers onto an endless first fabric to form a wet web; (b) dewatering the wet web to a consistency of about 10 percent to about 30 percent; (c) transferring the wet web to another fabric; (d) sandwiching the wet web between the second fabric and a support fabric, one of which utilizes the space and components formerly used in the bottom felt run of a tow press wet press machine; (e) dewatering the wet web to a consistency of greater than about 30 percent to about 40 percent using an air press that is adapted to cause a pressurized fluid at about 5 pounds per square inch gauge or greater to flow substantially through the web due to an integral seal formed between an air plenum and a collection device; (f) transferring the wet web back to the second fabric; (g) pressing the dewatered and wet web against the surface of a heated drying cylinder to at least partially dry the web; (h) drying the web to a final dryness.
In another aspect, the invention resides in a method for making a cellulosic web, comprising the steps of: (a) depositing an aqueous suspension of papermaking fibers onto an endless first fabric to form a wet web to make a wet web; (b) transferring the wet web to an endless second fabric; (c) sandwiching the wet web between the second fabric and a support fabric and dewatering the wet web to a consistency of about 10 percent to about 30 percent; (d) further dewatering the wet web to a consistency of greater than about 30 percent to about 40 percent using an air press that is adapted to cause a pressurized fluid at about 5 pounds per square inch gauge or greater to flow substantially through the web due to an integral seal formed between an air plenum and a collection device; (e) transferring the wet web back to the second fabric to give the web a bulk of about 8 cubic centimeter per gram or greater; (f) pressing the dewatered web against the surface of a heated drying cylinder with a fabric to preserve the bulk of about 8 cubic centimeter per gram or greater; and, (g) drying the web to a final dryness.
In yet another aspect, the invention resides in a method for making a cellulosic web, comprising the steps of: (a) depositing an aqueous suspension of papermaking fibers onto an endless first fabric to form a wet web; (b) transferring the wet web to an endless second fabric; (c) sandwiching the web between the second fabric and a support fabric; (d) passing the second and support fabrics with the wet web sandwiched therewithin between an air plenum and a collection device with the second fabric disposed between the wet web and the collection device; the air plenum and the collection device being operatively associated and adapted to create a pressure differential across the wet web of about 30 inches of mercury or greater and a stream of pressurized fluid through the wet web of about 10 standard cubic feet per minute per square inch or greater; (e) dewatering the wet web using the stream of pressurized fluid to a consistency of about 30 percent or greater; (e) pressing the wet web against the surface of a heated drying cylinder with the second fabric; and, (f) drying the web to a final dryness.
The term xe2x80x9cfirst fabricxe2x80x9d is used herein to refer to any fabric used in tissue making as described herein or known in the art, including, but not limited to, forming, molding, and other support fabrics used in making tissue. However, the first fabric is preferably a forming fabric. The term xe2x80x9csecond fabricxe2x80x9d is used herein to refer to any fabric used in tissue making as described herein or known in the art, including, but not limited to, forming, molding, and other support fabrics used in making tissue. However, the second fabric is preferably a molding fabric as described herein. Where the second fabric is a molding fabric, the resulting web is a molded web. The term xe2x80x9csupport fabricxe2x80x9d is used herein to refer to any fabric used in tissue making as described herein or known in the art, including, but not limited to, forming, molding, or any other fabric used in making tissue.
The terms xe2x80x9cintegral sealxe2x80x9d and xe2x80x9cintegrally sealedxe2x80x9d are used herein to refer to: the relationship between the air plenum and the wet web where the air plenum is operatively associated and in indirect contact with the web such that about 85 percent or greater of the air fed to the air plenum flows through the web when the air plenum is operated at a pressure differential across the web of about 30 inches of mercury or greater; and the relationship between the air plenum and the collection device where the air plenum is operatively associated and in indirect contact with the web and the collection device such that about 85 percent or greater of the air fed to the air plenum flows through the web into the collection device when the air plenum and collection device are operated at a pressure differential across the web of about 30 inches of mercury or greater.
The air press is able to dewater the wet web to very high consistencies due in large part to the high pressure differential established across the web and the resulting air flow through the web. In particular embodiments, for example, the air press can increase the consistency of the wet web by about 3 percent or greater, particularly about 5 percent or greater, such as from about 5 to about 20 percent, more particularly about 7 percent or greater, and more particularly still about 7 percent or greater, such as from about 7 to 20 percent. Thus, the consistency of the wet web upon exiting the air press may be about 25 percent or greater, about 26 percent or greater, about 27 percent or greater, about 28 percent or greater, about 29 percent or greater, and is desirably about 30 percent or greater, particularly about 31 percent or greater, more particularly about 32 percent or greater, such as from about 32 to about 42 percent, more particularly about 33 percent or greater, even more particularly about 34 percent or greater, such as from about 34 to about 42 percent, and still more particularly about 35 percent or greater.
By adding the integrally sealed air press dewatering step to the process, considerable improvements over the previously described existing processes can be achieved. First, and most importantly, a high enough consistency is achieved so that the process can operate at industrially useful speeds. As used herein, xe2x80x9chigh-speed operationxe2x80x9d or xe2x80x9cindustrially useful speedxe2x80x9d for a tissue machine refers to a machine speed at least as great as any one of the following values or ranges, in feet per minute: 1,000; 1,500; 2,000; 2,500; 3,000; 3,500; 4,000; 4,500; 5,000, 5,500; 6,000; 6,500; 7,000; 8,000; 9,000; 10,000, and a range having an upper and a lower limit of any of the above listed values. Further, molding the sheet at high consistencies significantly improves the ability of the sheet to retain its three-dimensionality and thus also significantly improves the resulting caliper of the sheet. As used herein, the term xe2x80x9ctexturedxe2x80x9d or xe2x80x9cthree-dimensionalxe2x80x9d as applied to the surface of a fabric, felt, or uncalendered paper web, indicates that the surface is not substantially smooth and coplanar. Additionally, the present machine configuration is amenable to incorporating a rush transfer step, which again results in a significant increase in bulk and absorbency relative to the existing wet pressing processes.
Optional steam showers or the like may be employed before the air press to increase the post air press consistency and/or to modify the cross-machine direction moisture profile of the web. Furthermore, higher consistencies may be achieved when machine speeds are relatively low and the dwell time in the air press is relatively high.
The pressure differential across the wet web provided by the air press may be about 25 inches of mercury or greater, such as from about 25 to about 120 inches of mercury, particularly about 35 inches of mercury or greater, such as from about 35 to about 60 inches of mercury, and more particularly from about 40 to about 50 inches of mercury. This may be achieved in part by an air plenum of the air press maintaining a fluid pressure on one side of the wet web of greater than 0 to about 60 pounds per square inch gauge (psig), particularly greater than 0 to about 30 psig, more particularly about 5 psig or greater, such as about 5 to about 30 psig, and more particularly still from about 5 to about 20 psig. The collection device of the air press desirably functions as a vacuum box operating at 0 to about 29 inches of mercury vacuum, particularly 0 to about 25 inches of mercury vacuum, particularly greater than 0 to about 25 inches of mercury vacuum, and more particularly from about 10 to about 20 inches of mercury vacuum, such as about 15 inches of mercury vacuum. The collection device desirably but not necessarily forms an integral seal with the air plenum and draws a vacuum to facilitate its function as a collection device for air and liquid. Both pressure levels within both the air plenum and the collection device are desirably monitored and controlled to predetermined levels.
Significantly, the pressurized fluid used in the air press is sealed from ambient air to create a substantial air flow through the web, which results in the tremendous dewatering capability of the air press. The flow of pressurized fluid through the air press is suitably from about 5 to about 500 standard cubic feet per minute (SCFM) per square inch of open area, particularly about 10 SCFM per square inch of open area or greater, such as from about 10 to about 200 SCFM per square inch of open area, and more particularly about 40 SCFM per square inch of open area or greater, such as from about 40 to about 120 SCFM per square inch of open area. Desirably, of the pressurized fluid supplied to the air plenum, 70 percent or greater, particularly 80 percent or greater, and more particularly 90 percent or greater, is drawn through the wet web into the vacuum box. For purposes of the present invention, the term xe2x80x9cstandard cubic feet per minutexe2x80x9d means cubic feet per minute measured at 14.7 pounds per square inch absolute and 60 degrees Fahrenheit (xc2x0 F.).
The terms xe2x80x9cairxe2x80x9d and xe2x80x9cpressurized fluidxe2x80x9d are used interchangeably herein to refer to any gaseous substance used in the air press to dewater the wet web. The gaseous substance suitably comprises air, steam or the like. Desirably, the pressurized fluid comprises air at ambient temperature, or air heated only by the process of pressurization to a temperature of about 300xc2x0 .F or less, more particularly about 150xc2x0 F. or less.
The wet web is desirably attached to the Yankee dryer or other heated drying cylinder surface in a manner that preserves a substantial portion of the texture imparted by previous treatments, especially the texture imparted by molding on three-dimensional fabrics. The conventional manner used to produce wet-pressed creped paper is inadequate for this purpose, for in that method, a pressure roll is used to dewater the wet web and to uniformly press the wet web into a dense, flat state. For the present invention, the conventional substantially smooth press felt is replaced with a textured material such as a foraminous fabric and desirably a throughdrying fabric. Tissue webs made according to the present method desirably have a bulk after being molded onto the three-dimensional fabric of about 8 cubic centimeters per gram (cc/g) or greater, particularly about 10 cc/g or greater, and more particularly about 12 cc/g or greater, and that bulk is maintained after being pressed onto the heated drying cylinder using the textured foraminous fabric.
For best results, significantly lower pressing pressures can be used as compared to conventional tissue making. Desirably, the zone of maximum load applied to the web should be about 400 psi or less, particularly about 350 psi or less, more particularly about 150 psi or less, such as between about 2 and about 50 psi, and most particularly about 30 psi or less, when averaged across any one-inch square region encompassing the point of maximum pressure. The pressing pressures measured in pounds per lineal inch (pli) at the point of maximum pressure are desirably about 400 pli or less, and particularly about 350 pli or less. Low-pressure application of a three-dimensional web structure onto a heated drying cylinder helps to maintain substantially uniform density in the dried web. Substantially uniform density is promoted by effectively dewatering the web with noncompressive means prior to the Yankee dryer attachment, and by selecting a foraminous fabric to contact the web against the dryer that is relatively free of high, inflexible protrusions that could apply high local pressure to the web. The fabric is desirably treated with an effective amount of a fabric release agent to promote detachment of the web from the fabric once the web contacts the dryer surface.
The absorbency of a tissue sheet may be characterized by its Absorbent Capacity and its Absorbent Rate. As used herein, xe2x80x9cAbsorbent Capacityxe2x80x9d is the maximum amount of distilled water which a sheet can absorb, expressed as grams of water per gram of sample sheet. More specifically, the Absorbent Capacity of a sample sheet can be measured by cutting a 4 inch by 4 inch (101.6 by 101.6 mm) sample of the dry sheet and weighing it to the nearest 0.01 gram. The sample is dropped onto the surface of a room temperature distilled water bath and left in the bath for 3 minutes. The sample is then removed using tongs or tweezers and suspended vertically using a 3-prong clamp to drain excess water. Each sample is allowed to drain for 3 minutes. The sample is then placed in a weighing dish by holding the weighing dish under the sample and releasing the clamp. The wet sample is weighed to the nearest 0.01 gram. The Absorbent Capacity is the wet weight of the sample minus the dry weight (the amount of water absorbed), divided by the dry weight of the sample. At least five representative samples of each product should be tested and the results averaged.
The xe2x80x9cAbsorbent Ratexe2x80x9d is the time it takes for a product to become thoroughly wetted out in distilled water. It is determined by dropping a pad comprised of twenty sheets, each measuring 2.5 inches by 2.5 inches (63.5 by 63.5 mm), onto the surface of a distilled water bath having a temperature of 30xc2x0 C. The elapsed time, in seconds, from the moment the sample hits the water until it is completely wetted (as determined visually) is the Absorbent Rate.
The present method is useful to make a variety of absorbent products, including facial tissue, bath tissue, towels, napkins, wipes, or the like. For purposes of the present invention, the terms xe2x80x9ctissuexe2x80x9d or xe2x80x9ctissue productsxe2x80x9d are used generally to describe such product structures, and the term xe2x80x9ccellulosic webxe2x80x9d is used to broadly refer to webs comprising or consisting of cellulosic fibers regardless of the finished product structure.
Many fiber types may be used for the present invention including hardwood or softwoods, straw, flax, milkweed seed floss fibers, abaca, hemp, kenaf, bagasse, cotton, reed, and the like. All known papermaking fibers may be used, including bleached and unbleached fibers, fibers of natural origin (including wood fiber and other cellulosic fibers, cellulose derivatives, and chemically stiffened or crosslinked fibers) or synthetic fibers (synthetic papermaking fibers include certain forms of fibers made from polypropylene, acrylic, aramids, acetates, and the like), virgin and recovered or recycled fibers, hardwood and softwood, and fibers that have been mechanically pulped (e.g., groundwood), chemically pulped (including but not limited to the kraft and sulfite pulping processes), thermomechanically pulped, chemithermomechanically pulped, and the like. The mixtures of any subset of the above mentioned or related fiber classes may be used. The fibers can be prepared in a multiplicity of ways known to be advantageous in the art. Useful methods of preparing fibers include dispersion to impart curl and improved drying properties, such as disclosed in U.S. Pat. No. 5,348,620 issued Sep. 20, 1994 and U.S. Pat. No. 5,501,768 issued Mar. 26, 1996, both to M. A. Hermans et al.
Chemical additives may be also be used and may be added to the original fibers, to the fibrous slurry or added on the web during or after production. Such additives include opacifiers, pigments, wet strength agents, dry strength agents, softeners, emollients, humectants, viricides, bactericides, buffers, waxes, fluoropolymers, odor control materials and deodorants, zeolites, dyes, fluorescent dyes or whiteners, perfumes, debonders, vegetable and mineral oils, humectants, sizing agents, superabsorbents, surfactants, moisturizers, UV blockers, antibiotic agents, lotions, fungicides, preservatives, aloe-vera extract, vitamin E, or the like. The application of chemical additives need not be uniform, but may vary in location and from side to side in the tissue. Hydrophobic material deposited on a portion of the surface of the web may be used to enhance properties of the web.
The headbox may be stratified to permit production of a multilayered structure from a single headbox jet in the formation of a web. In particular embodiments, the web is produced with a stratified or layered headbox to preferentially deposit shorter fibers on one side of the web for improved softness, with relatively longer fibers on the other side of the web or in an interior layer of a web having three or more layers. The web is desirably formed on an endless loop of foraminous forming fabric which permits drainage of the liquid and partial dewatering of the web.
Numerous features and advantages of the present invention will appear from the following description. In the description, reference is made to the accompanying drawings which illustrate preferred embodiments of the invention. Such embodiments do not represent the full scope of the invention. Reference should therefore be made to the claims herein for interpreting the full scope of the invention.